1. Field of the Invention
The present invention relates to a torque direction detecting device for a brushless motor.
2. Description of the Related Art
A brushless motors has been used in various control systems such as an electric power steering control system because of its simple mechanical structure and excellent controlling performance. In order to drive such a brushless motor, rotation position of the brushless motor's rotor is detected by a rotation position detector such as a resolver, which generates a rotating magnetic field. A control system and a three-phase bridge circuit are provided to drive the brushless motor according to the rotation position signal. The rotation position detector also detects the direction of the rotor torque and an abnormal operation of the brushless motor.
The resolver is a rotary transformer, which has a pair of stator windings and a rotor winding. The pair of stator windings are disposed at positions of a stator that are 90 degrees in mechanical angle different from each other. The rotor winding magnetically interlinks the pair of stator windings to generate a signal whose amplitude is a function of the rotor's rotation position or a rotation angle relative to the stator. Therefore, the resolver generates two kinds of signals, which are modulated by a sine function of the rotation angle of the rotor and a cosine function of the rotation angle of the rotor, based on a field exciting signal that is inputted to the resolver.
JP-A-2003-235285 or its counterpart US-2003-0151383A1 discloses a torque direction detecting device for a brushless motor in which the torque direction is determined according to a difference in amount between two phase currents at a suitable timing that is obtained from the electric angle of the brushless motor, which is based on output signals of a resolver.
When a field exciting signal sin ωt, as shown in FIG. 3A, is inputted to a resolver, a SIN output signal sin θ×sin ωt, as shown in FIG. 3B, and a COS output signal cos θ×sin ωt, as shown in FIG. 3C, are outputted. When sin ωt becomes 1, the SIN signal becomes sin θ and the COS signal becomes cos θ, as shown in FIG. 3D. One cycle or 360 degrees in electric angle of the SIN and COS output signals is divided into 8 sections T1–T8 in such that the electric angle is set 0 when sin θ is 0 and cos θ is 1.
In order to divide the one cycle into 8 timing sections as shown in FIG. 6B, the torque direction detecting device disclosed in JP-A-2003-235285 includes at least five comparators 33–36, 47 for providing one of three Bit signals (e.g. Bit 2) in addition to comparators 31, 32 for providing two Bit signals (e.g. Bit 0, Bit 1), as shown in FIG. 6A, because of generating absolute values. However, these kinds of circuits makes the torque direction detecting device complicated and expensive.